Field of the Invention
The present invention relates to an X-ray Talbot interferometer.
Description of the Related Art
An X-ray phase imaging technique is an imaging technique using phase changes that occur as X-rays pass through an object. Examples of X-ray phase imaging techniques that have been proposed include a Talbot interferometer described in Japanese Patent No. 5162453. The Talbot interferometer typically includes two or three gratings each having a periodic structure. Of these gratings, a grating normally disposed near the object may be referred to as a beam splitter grating, a grating normally disposed near a detector may be referred to as an analyzer grating, and a grating normally disposed near an X-ray source may be referred to as a source grating. These gratings may each be either a grating having a one-dimensional periodic pattern, or a grating having a two-dimensional periodic pattern. The detector is normally one that is capable of acquiring a two-dimensional intensity distribution of X-rays incident on the detection surface of the detector.
The beam splitter grating is typically a phase-modulation transmissive diffraction grating. X-rays incident on the beam splitter grating are diffracted by the periodic structure of the grating to form an interference pattern (also called a self-image of the grating) at a predetermined position by a so-called Talbot effect. Since the interference pattern is deformed reflecting, for example, phase changes that occur as X-rays pass through the object, information about the shape and internal structure of the object can be obtained by measuring and analyzing the intensity distribution of the interference pattern.
The analyzer grating is typically a grating that has a periodic transmittance distribution, because of the periodic arrangement of X-ray transmitting portions and X-ray shielding portions. The analyzer grating is disposed at the position of the interference pattern, and thus is used for the purpose of producing moire in the intensity distribution of X-rays transmitted through the grating. The moire reflects the deformation of the interference pattern, and the period of the moire can be increased infinitely. Therefore, even when the spatial resolution of the detector to be used is not high enough to allow direct detection of the interference pattern, information about the interference pattern can be indirectly obtained by detecting moire having a large pattern period.
Like the typical analyzer grating described above, the source grating is a grating having a structure where X-ray transmitting portions and X-ray shielding portions are periodically arranged. The source grating is normally disposed near an X-ray emitting spot in the X-ray source (X-ray generator), and used for the purpose of virtually forming an array of linear X-ray-emitting portions (or small X-ray-emitting spots in the case of a two-dimensional grating). A plurality of interference patterns formed by X-rays emitted from the linear X-ray-emitting portions are superimposed while displaced from each other by an integral multiple of the pattern period, in the absence of any object in the X-ray path. Thus, there is no pattern loss even when many interference patterns are superimposed, and it is possible to form a periodic pattern having generally high X-ray intensity and fringe visibility. To achieve the superimposition described above, the grating period of each grating and the distance between gratings need to be designed to meet certain conditions. A Talbot interferometer using the source grating described above may be specifically referred to as a Talbot-Lau interferometer. A Talbot interferometer using the source grating described above is disclosed in Japanese Patent No. 5162453. Hereinafter, the term “Talbot interferometer” includes a Talbot-Lau interferometer.
In an imaging technique using a Talbot interferometer, detection of interference patterns or moire patterns by a detector is generally followed by analysis of the detected patterns for conversion to a more useful image. Another imaging technique is known, in which a positional relation between gratings during imaging is designed to meet specific conditions, so that a detected special moire image can be directly used as an object image. For example, in imaging techniques described in Japanese Patent No. 5162453 and “Phase-contrast imaging using a scanning-double-grating configuration”, OPTICS EXPRESS (US), 2008, Vol. 16, No. 8, pp. 5849-5867, by Y. I. Nesterets and S. W. Wilkins, imaging is performed by forming moire with a very large (ideally infinite) period, using an interferometer having a grating arrangement where an interference pattern and an analyzer grating pattern are precisely the same in direction and pitch. Here, a periodic pattern having the same period as the interference pattern may be formed on a detector, or no periodic pattern may be formed on the detector. In either case, an X-ray intensity acquired by each of a plurality of pixels of the detector is substantially the same. That is, the intensity distribution acquired by the detector is substantially uniform.
The document by Y. I. Nesterets and S. W. Wilkins describes a technique in which the relative positions of gratings are adjusted such that the intensity of X-rays transmitted through the gratings is minimized in the imaging region. Then, with the beam splitter grating and the analyzer grating fixed to each other, imaging is performed while both the gratings are being scanned at the same time. With this technique, the detector can acquire an image which strongly reflects not only absorption information of the object, but also scattering information (small-angle X-ray scattering power by microparticles, fine fibers, edges of structures, etc.) along the periodic direction of the interference pattern. In this technique, differential phase information along the periodic direction of the interference pattern is strongly reflected in the image if differential phase values are large and the local phase of interference fringes is sufficiently significantly shifted, but is not strongly reflected in the image if the local phase shift is not significant enough. In other words, the image obtained by the detector does not have high sensitivity to differential phase information. Even when imaging is performed without scanning of the two gratings, the resulting image will be substantially the same if the spatial resolution of the imaging system is not particularly high. This imaging technique which performs imaging, with the grating positions adjusted to minimize the intensity of X-rays transmitted through the gratings, is similar to a so-called dark-field technique in an optical microscope. In the present specification, an imaging technique which performs imaging with such grating positions (not based on the assumption of scanning of gratings) may be referred to as a dark-field technique. Conversely, an imaging technique which performs imaging, with the relative positions of gratings adjusted such that the intensity of X-rays transmitted through the gratings is maximized in the imaging region, may be referred to as a bright-field technique in the present specification.
Japanese Patent No. 5162453 describes a technique which performs imaging, with the relative positions of gratings adjusted such that the intensity of X-rays transmitted through the gratings is about the average of maximum and minimum values throughout the imaging region. In the present specification, this imaging technique may be referred to as an intermediate technique. With the intermediate technique, the detector can acquire an image which strongly reflects not only absorption information of the object, but also differential phase information (spatial differential values of the phase distribution of X-rays transmitted through the object) along the periodic direction of the interference pattern. However, with the intermediate technique, the detector can acquire very little scattering information of the object.
In the present specification, a phase distribution of X-rays transmitted through the object, a differential phase distribution obtained by spatially differentiating the phase distribution, and a secondary differential phase distribution obtained by differentiating the differential phase distribution in the same direction may be collectively referred to as phase information of the object.
With the use of a Talbot interferometer, when imaging is performed under conditions where the intensity distribution of X-rays transmitted through gratings is uniform in the imaging field, it is possible to acquire an image which strongly reflects not only the absorption information of the object, but also the phase information and the scattering information of the object. However, as described above, a Talbot interferometer which performs the intermediate technique is unable to acquire scattering information of the object. Also, a Talbot interferometer which performs the dark-field technique has low sensitivity to phase information of the object.